Various types of pumps are utilized in fluid transporting systems in order to develop and maintain a desired amount of flow energy in the fluid. Many of these pumps require for their operation at least one rotatable shaft to drive a mechanical energy-transferring device such as a piston, impeller, or gear. Typically, the rotational power or torque transmitted to the shaft is generated in a motor disposed in remote relation to the pump housing. Thus, a portion of the shaft necessarily extends outside the housing through, for example, a bore in a wall of the housing, for direct or indirect linkage to the motor. The shaft is supported or mounted in the housing, but must be free to rotate at the interface of the housing and shaft in accordance with the operation of the pump.
A clearance of operationally-significant magnitude therefore exists between the bore of the housing wall and the shaft, even in a case where a bushing or like element is employed at the shaft housing or pump/atmosphere interface. It is recognized that over the range of operating pressures of the pump, this clearance presents a potential leakage point. Depending on the direction of the pressure gradient between the interior of the pump housing and the atmosphere, the leakage point may be characterized by fluid leaking out of the pump or air infiltrating into the pump. The leakage may contribute to a variety of undesirable conditions, including reduced pump efficiency, reduced economic life of the pump and related components, increased maintenance costs, and contamination or non-uniformity of the fluid being pumped. Accordingly, it is well understood that the pump must include some means for sealing the shaft at the interface.
The approach taken in the design of the shaft seal is especially critical in the context of gear pumps, which are utilized in a number of well-known applications to meter and discharge various types of fluids. A gear pump may generally be described as being a rotary, positive displacement pump. In its most basic design, the gear pump includes a pair of intermeshing spur, single-helical or double-helical (i.e., herringbone) gears disposed in a housing having narrow internal dimensional tolerances. One gear serves as the driving gear and is rotatable with a drive shaft, i.e., the shaft powered by a motor. The other gear serves as the driven gear and is rotatable on an idler shaft. The shafts are mounted in journal bearings on each side of gears. In operation, the gears create a pressure differential between a suction side and a discharge side of the gear pump housing. The working fluid is drawn into the housing at the suction side, is carried by the teeth of each gear in spaces defined by the teeth and one or more internal surfaces of the housing, and is squeezed out on the discharge side. This design results in a relatively constant rate of fluid flow with a minimum of drifting or slippage. The flow rate is dependent on gear rotational speed, but is largely unaffected by viscosity variations and pressure differential variations across the gear pump.
The performance characteristics of the gear pump make it especially useful in the processing of high-shear polymers such as rubber, PVC, and EDPM, where pressure, volume and uniformity of the flowing material must be controlled. For example, the gear pump may be used to transport synthesis polymeric material from a reaction vessel. The gear pump may also be used in connection with an extruder. A typical extruder includes an elongate barrel containing a rotating auger or screw. A hopper feeds pellets or granules of the polymeric material to the barrel, where the material is heated and melted as it is forced along the length of the barrel by the screw. In such an application, the gear pump is installed between the extruder and an extrusion die to pressurize and meter the polymer melt flow, and to dampen any pressure fluctuations or surges caused by the rotating screw of the extruder. Because the gear pump moves fluid more efficiently than the extruder and reduces the load on the extruder, the gear pump itself can be used to develop the high pressure needed in the fluid line. This enables the discharge pressure of the extruder to be separately adjusted to a reduced level in better accord with the extruder's own optimal operating point. Finally, the gear pump may be installed in line with two or more extruders as part of a compounding or mixing process to obtain similar advantages.
In view of the foregoing, it is readily apparent that the gear pump may produce not only a high pressure differential between the inlet and outlet fluid conduits communicating with the gear pump, but also a high pressure differential between the interior of the gear pump and the atmosphere. Thus, the problem of leakage in gear pumps may be potentially significant.
The leakage problem is further exacerbated when the gear pump is used to process viscous fluids. For example, in polymeric material processing the bearings selected for the gear pump are typically hydrodynamic and self-lubricating. That is, instead of using a separate lubrication method such as a forced oil circulation system, the gear pump and bearings are designed with flow paths for diverting a portion of the incoming polymer melt flow and circulating that portion between the bearings and shafts prior to discharge from the gear pump. The radial clearance provided in the bearing permits a wedgeshaped polymeric film to develop between the journal and the bearing as the shaft rotates. As a result, a hydrodynamic pressure is generated in the film that is sufficient to float the journal portions of the shafts and support the loads applied to them. And since the journal portion of the rotating shaft does work on the polymeric film and induces shear stresses therein, the frictional heat energy produced raises the film temperature. Consequently, the heated and pressurized polymer melt flowing in the vicinity of the shaft/housing interface has a high tendency to leak out from the pump.
Previous sealing solutions have not adequately controlled the leakage problem observed in gear pumps. In one application typical of the prior art, the sealing means took the form of a packing seal. A packing seal is constructed of one or more layers, windings or gaskets constructed of packing material such as graphite-impregnated cotton. The packing material is compressed within a packer or stuffing box. The stuffing box is usually disposed adjacent to the main pump housing. The main shaft of the gear pump extends outside the housing and through the stuffing box, such that the compressed packing material is squeezed against the shaft.
Apart from its general ineffectiveness in environments marked by high pressure differentials, the packing seal suffers from several other problems The compressed packing material, although treated with graphite, is nonetheless abrasive enough to produce substantial frictional contact with the shaft and thereby accelerate wear and deterioration of the shaft as well as the packing material itself, inviting frequent replacement of both. Additionally, the excessive frictional contact engendered by the packing material causes the pump to work harder, which lowers output and efficiency.
An attempt to improve the utility of the packing seal in the context of polymer processing is disclosed in U.S. Pat. No. 4,515,512 to Hertell et al. The gear pump disclosed in the Hertell patent includes a stuffing box attached to an end wall of the main pump housing. The stuffing box is thus adjacent to and outside of the housing. The drive shaft of the gear pump extends through a bore in the end wall of the housing, through the stuffing box, and to the outside. There is no seal directly located in the clearance or gap created between the shaft and the bore of the gear pump housing end wall Accordingly, the fluid being pumped has a relatively unrestricted path by which to flow through the gap and into the stuffing box.
The Hertell patent provides two sets of gaskets, which are packed within the stuffing box in annular disposition around the shaft and the inner contour of the stuffing box. An annular cavity in the stuffing box separates the two gasket sets. A plurality of springs are circumferentially spaced in the annular cavity between the between the two gasket sets. The end of the stuffing box opposite the main pump housing is capped with a threaded flange annularly disposed around the shaft. Adjustment of the flange maintains axial compression of the gaskets in the direction of the gear housing, thereby maintaining frictional contact between the gaskets and the shaft. Within the annular cavity, the springs provide a biasing force to maintain a volume in the cavity between the two gaskets sets, as well as assist in compressing the gaskets. An inlet an outlet tube are placed in communication with the cavity and lead to a remote solvent reservoir, which stores a polymer solvent such as glycol. This arrangement serves to circulate and cool the solvent in the annular cavity.
In operation, some of the pressurized polymeric material in the housing of the gear pump in the Hertell patent tends to leak through the gap in the end wall in the direction of the stuffing box. However, the pump is configured with a bypass line such that the pressure in the gap is essentially equalized to the pressure on the suction side of the pump. This creates a pressure gradient in the direction of the stuffing box to the gear pump housing, so that polymer solvent tends to travel from the annular cavity of the stuffing box toward the housing. In this manner, it is intended that the solvent meet the leaking polymer and dissolve it.
It should be apparent from the foregoing that the concept disclosed in the Hertell patent is primarily directed at protecting the packing seal from leaking polymeric material by incorporating a complex and burdensome polymer solvent circulation system into the gear pump. That is, this concept does not focus on preventing leakage of fluid from the pump housing. In practice, the concept may improve the life of the packing material of the seal, but does not resolve the afore-described problems associated with the packing seal itself. Moreover, the solvent circulation system introduces additional problems. For instance, the Hertell patent acknowledges that, due to the pressure gradient, some of the solvent supplied may reach the interior of the gear pump and be discharged with the polymer melt flow. Such a result is clearly undesirable where even moderate quality control of the polymer product is specified. Also, the range of use of the Hertell system is limited, as many high-pressure/high-viscosity/high-temperature applications could be expected to overcome the capacity of the solvent system to prevent polymeric material from flooding the stuffing box, degrading or overwhelming the packing material, and leaking to the atmosphere. Another approach to sealing a gear pump operating in a highly viscous environment is disclosed in U.S. Pat. No. 4,699,575 to Geisel et al., which avoid use of a stuffing box. In the Geisel patent, a plurality of annular bushings constructed of a resilient plastic are press-fitted onto the drive and idler shafts of an adhesive gear pump, at locations between each gear and each journal bearing of the gear pump. The gear pump is configured with means for circulating an incompressible lubricant grease at high pressure throughout the gear pump, and through gaps located in proximity to the plastic bushings. The circulation means requires, among other things, several grease fittings for charging the circulation system, several internal passages within the gear pump, and high-pressure outlet relief valves leading to the atmosphere. According to the Geisel patent, the adhesive flowing through the gear pump is prevented from creeping past the bushings because the gaps are kept continuously filled with the incompressible grease. This approach presents many of the same disadvantages as described in regard to the Hartell patent, in that it specifies a system for circulating an additional material through the gear pump and accordingly introduces unnecessary complexities.
The first valid approach toward solving, rather than mitigating, the leakage problem in polymer processing applications is believed to be disclosed in U.S. Pat. No. 4,336,213 to Fox. In the gear pump disclosed therein, a seal is provided directly at the housing/shaft interface, and uses the polymeric material itself to complete the seal. The seal includes a cylindrical sleeve that is inserted onto the portion of the shaft extending beyond the pump housing. The seal member has a flange at the end of the sleeve opposite the pump housing. A plurality of holes are circumferentially disposed around an annular shoulder portion of the flange, through which bolts may be inserted to tightly secure the seal to the housing in annular disposition with the shaft. When inserted onto the shaft, the cylindrical inner surface of the sleeve abuts the outer surface of the shaft. Accordingly, the inner surface of the sleeve and the outer surface of the shaft together define a clearance or gap which becomes the potential leakage point for the gear pump.
The seal in the Fox patent is characterized in part by the fact that a shallow helical channel is formed on the inner surface of the sleeve. The helical channel extends substantially along the entire length of the inner surface The orientation or "hand" of the helical path taken by the channel is opposite to that of the shaft rotation. Thus, during operation of the gear pump, polymeric material entering the clearance between the sleeve and shaft tends to travel in the helical channel. However, given the opposite orientation of the helix, the leaking material is effectively pumped back toward the interior of the pump housing and thus is prevented from leaking to the outside. In essence, the configuration of the sleeve, flanged and bolted to the housing, provides a mechanical seal while the polymeric material opposed by the helical channel provides a viscous, relatively static seal. Furthermore, the existence of the polymeric material in the clearance significantly reduces friction therein. Accordingly, this design has been highly effective as a seal for gear pumps operating over a considerable range of pressures, temperatures and viscosities.
In U.S. Pat. No. 4,471,963 to Airhart, an attempt was made to improve upon the design disclosed in the Fox patent. As in the Fox patent, the seal provided in the Airhart patent includes a cylindrical sleeve that is flanged and bolted to the housing of the gear pump. Two helical channels are formed on the inner surface of the sleeve and are axially separated by a relatively deep and wide annular cavity. The first helical channel begins at a point proximate to the pump housing and terminates in fluid communication with the annular cavity. On the opposite side of the annular cavity farthest from the housing, the second helical channel communicates with the annular cavity and terminates at a point proximate to the outer end of the sleeve. The orientation of the first helical channel is the same as that of the rotating shaft, and hence polymeric material leaking from the gear pump had a high tendency to flow through the first helical channel and accumulate in the annular cavity. On the other hand, the second helical channel has an opposite orientation, such that it impedes outwardly axial flow of polymeric material beyond the annular cavity.
The seal in the Airhart patent is characterized in that means are provided for actively cooling the polymeric material accumulated in the annular cavity so as to create a polymeric plug. Two bores are drilled at diametrically opposite sides of the flange and communicate with an annular passageway formed within the solid cross-sectional portion of the cylindrical sleeve of the seal. The bores are connected via tubing to a circulation system. During operation of the pump, water or other coolant is circulated through the bores and the annular passageway to carry heat away from the polymeric material present in the seal, thereby solidifying the polymeric material and forming the plug.
The approach for improving the helically-channeled seal disclosed in Fox by active cooling is at first glance attractive. However, as in the case of the Hertell and Geisel patents, the seal in the Airhart patent requires external equipment and conduits to circulate an additional fluid through the pump. This adds to the cost and complexity of the gear pump, and introduces additional areas of maintenance.
The present invention is therefore provided to solve these and other problems associated with the prevention of leakage of rotating shafts in general, and specifically with the prevention of leakage at the shaft/housing interface of gear pumps operating in polymer processing applications.